JPS639314B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a video tape recorder (hereinafter simply referred to as
It's called a VTR. ) etc., the angle modulation signal reproducing circuit for reproducing the angle modulation signal recorded on the recording medium is particularly concerned with preventing the so-called inversion phenomenon caused by sudden changes in the signal level and reducing S/N deterioration. An object of the present invention is to provide an angle modulation signal reproducing circuit which can obtain a high quality reproduced output signal without any distortion.

In general, magnetic recording and reproducing systems operate under conditions such as home VTRs, where the relative speed between the magnetic recording medium and the magneto-electric transducer is low and the track width is extremely narrow. By handling with emphasis, the S/N ratio can be improved. but,
When recording and reproducing signals at high deviation and high emphasis, images in which the brightness signal level of the video signal changes rapidly, such as images that change from black to white in the horizontal scanning direction of the screen, are played back. In this case, as shown in FIG. 1, a reversal phenomenon occurs in which a horizontal black line 4 appears in the white area 2 from the boundary line 3 where the black area 1 changes to the white area 2. It may be accompanied.

The reason why the above reversal phenomenon occurs can be electrically explained as follows. In other words, a video signal as shown in FIG. 2A, which is an image that changes from black to white in the horizontal scanning direction of the screen, generally has the following characteristics:
Since the signal is pre-emphasized in a VTR, overshoot occurs in the portion where the black level changes to the white level, as shown in FIG. 2B. When such a recorded video signal is angularly modulated by an angle modulator, the carrier level at the overshoot section increases, so in a VTR using the low-frequency carrier vestigial sideband FM method, the carrier level at this section increases. The level is decreasing. Additionally, due to transient distortion in the signal transmission system, phase shift due to high-pass filters, etc., an amplitude modulation component is generated in the overshoot area, and as shown in Figure 2C, the amplitude modulation component is Therefore, the shift of the zero cross of the reproduction angle modulation signal, that is, the peak or valley of the carrier no longer intersects on the time axis. Therefore, if the reproduced angle modulation signal is waveform-shaped through a hard limiter in the signal reproduction system, pulses will be deleted as shown in FIG. 2D, making faithful demodulation impossible, and as shown in FIG. 2E. In this way, an inversion phenomenon occurs at the transition portion of the final reproduced video signal from the black level to the white level, which is regarded as the black level.

In order to prevent such an inversion phenomenon, it is sufficient to emphasize the high-frequency components of the carrier before limiting the amplitude of the reproduced angle modulation signal. In other words, the above-mentioned high-frequency emphasis increases the carrier level of the overshoot portion of the reproduced angle modulation signal, causing the signal to cross the time axis, resulting in pulse deletion even if amplitude limiting is performed at a later stage. Things will go away.

However, in general, the low carrier vestigial sideband FM method
In VTR, the S/N of the high frequency component of the carrier is poor.
The more this high-frequency component is emphasized, the more the S/N of the reproduced video signal obtained by demodulation deteriorates. In other words, if the above-mentioned inversion phenomenon is prevented, S/N deterioration occurs, and S/N decreases.
There is a contradiction in that if N is increased, it becomes impossible to prevent the reversal phenomenon.

A circuit with the configuration shown in Figure 3 has been proposed as a magnetic recording and reproducing system to resolve the above-mentioned contradictions, prevent the reversal phenomenon without S/N deterioration, and obtain a high-quality demodulated output signal. has been done.

In the magnetic recording and reproducing system shown in FIG. 3, an input video signal (Y _io signal) having a frequency distribution state as shown in FIG. 4A is supplied to the signal input terminal 10, and this input video signal (Y _io signal) Angle modulation circuit 1
1 and has an upper sideband and a lower sideband as shown in FIG. 4B.
-FM signal) and the recording magnetic head 12
The data is recorded on the magnetic tape 13 via the magnetic tape 13. here,
It is assumed that the signal input terminal 10 is supplied with an input video signal (Y _io signal) having the waveform shown in FIG. 2B described above. The angle modulation signal (Y-FM signal) recorded on the magnetic tape 13 is detected by the reproduction magnetic head 14, and as shown in FIG. _put -FM signal) through the regenerative amplifier 15. The waveform of the regenerative angle modulation signal (Y _put -FM signal) obtained through the regenerative amplifier 15 is as follows:
It is as shown in FIG. 2C above. The above reproduced angle modulation signal (Y _put -FM signal) is generated by a soft limiter circuit 1 with a limiter gain of about 10 to 12 dB.
6, the fourth
As shown in Figure D, the upper sideband is reproduced by the lower sideband component of the Y _put -FM signal. In this way, the reproduced angle modulation signal (Y- _put -FM signal) in which the upper sideband has been reproduced has a waveform with the zero-crossing portion corrected as shown in Figure 2F, and further as shown in Figure 4E. Since the high-frequency boost circuit 17 having the frequency characteristics shown in FIG. 4 converts the high-frequency signal components into an emphasized frequency distribution state as shown in FIG. Angle demodulation circuit 19 via
supplied to Here, the regenerative angle modulation signal (Y _put -FM signal) obtained via the regenerative amplifier 15
Although the S/N ratio of the upper sideband component is poor, the upper sideband of the reproduction angle modulation signal ( _Yput -FM' signal) obtained via the soft limiter circuit 16 _is
It is reproduced with good S/N by the lower sideband component with good S/N in the FM signal. Therefore, even if the high frequency component is emphasized by the high frequency enhancement circuit 17, signal S/N deterioration will not occur. That is, for example, if an input video signal ( _Yio signal) having a waveform as shown in FIG. For the reproduced angle modulation signal (Y _put -FM signal) shown in C, the amplitude is limited by the soft limiter circuit 16 to obtain a signal (Y _put -FM' signal) with a waveform shown in Fig. 2D. , Furthermore, for the high frequency component of this signal, the high frequency amplifier circuit 1
7 to emphasize the high frequency range to ensure correct zero crossing as shown in FIG. 2G, and then amplitude limitation is performed by the hard limiter 18. The output signal obtained through the hard limiter 18 can be correctly demodulated by the demodulator 19 without pulse deletion, as shown in FIG. 2H. FIG. 2 shows the waveform of the demodulated output signal obtained by angle demodulation by the demodulator 19.

By the way, in the magnetic recording and reproducing system having the configuration shown in FIG. The third harmonic component of the carrier of the reproduction angle modulation signal is emphasized by the high frequency enhancement circuit 17,
In particular, if the gain of the high frequency enhancement circuit 17 is increased too much, the third harmonic component may cross zero, causing an inversion phenomenon.

For example, when recording and reproducing an input video signal that changes from a white level to a black level and is accompanied by undershoot due to pre-emphasis, as shown in FIG. The reproduction angle modulation signal shown in FIG. 5 is amplitude-controlled via the soft limiter circuit 16 as shown in FIG. Since the third harmonic component of the carrier of the reproduction angle modulation signal is harmonized, if this signal is amplitude limited by the hard limiter circuit 18, an unnecessary pulse P _e is generated as shown in FIG. 5E. Therefore,
Since the unnecessary pulse P _e is determined to be at the white level by the demodulator 19, an inversion phenomenon due to white overmodulation will occur.

Therefore, in view of the above-mentioned problems in magnetic recording and reproducing systems, the present invention reliably prevents the reversal phenomenon caused by black overmodulation and white overmodulation without S/N deterioration. An object of the present invention is to provide an angle modulation signal reproducing circuit that can obtain a high quality reproduced output signal.

Before explaining the embodiments of the present invention, the third
The following describes the ideas for the high frequency enhancement circuit 17 in the magnetic recording/reproducing system shown in the figure. That is, in general, the high frequency enhancement circuit 17 in a magnetic recording/reproducing system is designed by devising the phase rotation of the signal, etc.
A so-called aperture compensation circuit using a delay circuit 21 and having a configuration as shown in FIG. 6 is used.
The delay circuit 21 has a delay characteristic of a unit delay time τ. The reproduced angle modulated original signal e _io (a waveform diagram is shown in FIG. 7A) supplied via the matching resistor 22 is delayed by a delay time τ by the delay circuit 21 and then sent to the signal addition circuit shown in FIG. 7B. 23
As shown in FIG. 8, the circuit is composed of two differentially connected transistors 24 and 25, and converts the composite signal e ₀ into the signal 1 with a gain of 1/2. By adding signals in opposite phase to the second reflected wave signal e, a reproduced angle modulation signal _eput having a waveform as shown in FIG. 7F is output. In the reproduction angle modulation signal e _put obtained in this way, the fundamental waveform component is suppressed and the third harmonic component is emphasized. If the third harmonic component is too emphasized, the broken line in FIG. As shown in the figure, white overmodulation occurs due to zero crossing. In order to prevent the zero cross from occurring, the unnecessary portion due to the third harmonic component can be removed by limiting the amplitude of the composite signal e ₀ as shown in FIG. 7E.

Therefore, the present invention imposes an amplitude limit on a signal with an enhanced third harmonic component obtained by adding the reproduction angle modulated original signal and a signal delayed by 2τ time, which is twice the unit delay time τ. The amplitude-limited signal and the signal delayed by a unit delay time from the original signal are added in opposite phases to obtain a reproduced angle modulation signal with enhanced high-frequency components. Therefore, the zero crossing caused by the third harmonic component is improved and the reversal phenomenon is prevented.

That is, the embodiment shown in FIG.
This is a modified version of the high frequency enhancement circuit shown in the figure.
Transistor 2 configuring the signal addition circuit 23
4 and 25, two diodes 3 are connected to the base of the transistor 25 to which the composite signal e ₀ is supplied.
Amplitude limiting circuit 33 formed by connecting 1 and 32 in opposite directions.
are connected for AC. In this embodiment, each component other than the amplitude limiting circuit has the same configuration as the high frequency enhancement circuit shown in FIG. 8 above.
Identical components are designated by the same numbers in the drawings, and detailed description thereof will be omitted. An example of such a configuration can be expressed as an equivalent block diagram as shown in FIG. 10. In an embodiment with such a configuration, if the transfer function of the connection point between the matching resistor 22 and the delay circuit 21 from which the composite signal e ₀ is obtained is determined, the composite signal e ₀ is a combination of the original signal e _io and the twice reflected wave. Since it consists of the signal e ₂ 〓, it is expressed as e〓 ₀ = e〓 _io (1 + e ^-j2 〓〓) = e〓 _io (1 + cos2ωτ−j sin2τ), and the gain G is G = |e ₀ /e _io |=√2(1+2) The phase Q is: Q=tan ^-1 -sin2ωτ/1+cos2ωτ= ^{tan -1} 2ω
τ/(1+cos2ωτ) ²・[(−sin2ωτ)′・(1/
1+cos2ωτ) −(−sin2ωτ)・(1/1+cos2ωτ)′]=1/
1+sin ² 2ωτ/(1+cos2ωτ) ²・[2τcos2ωτ/
1+cos2ωτ +2τ・sin ² 2ωτ/(1+cos2ωτ) ² ]=(1+cos
2ωτ) ² / (1 + cos2ωτ) ² + sin ² 2ωτ・[2τ・(c
os2ωτ+1)/(1+cos2ωτ) ² ] =2τ(1+cos2ωτ)/2(1+cos2ωτ), which makes τ constant regardless of frequency.

In this embodiment, the amplitude of the composite signal e ₀ obtained by adding the twice-reflected wave signal e ₂ 〓 by the delay circuit ₂₁ and the original signal e _io is limited in amplitude. The peak and dip portions of the signal level are removed by the third harmonic component. Therefore, in the signal addition circuit, the limiter output signal e _l having the waveform as shown in FIG. 7E and the above-mentioned once reflected wave signal e are added, and a zero cross is obtained as shown by the solid line in FIG. 7F. Unaccompanied angle modulation playback output signal
You can get e _put .

As in the above embodiment, two diodes 31,
Instead of using the amplitude limiting circuit 33 formed by connecting 32 transistors in the opposite direction, the amplitude can be limited by using the voltage V _BE between the base and emitter of the transistor.
For example, in the embodiment shown in FIG. 11, two transistors 24' and 2 are used as the signal addition circuit 23'.
Using a differential amplifier configured by commonly connecting each emitter of 5', the amplitude of the composite signal _e0 is limited by the base-emitter voltage _VBE1 of the transistor 25' whose base is supplied with the composite signal _e0 . Can be done.
Note that in FIG. 11, the transistor 26 is a switching transistor that is turned on and off depending on the operating state of the magnetic recording and reproducing system, and is turned on in the EE mode and the REC mode.
An operation is performed to stop the supply of the composite signal e ₀ to the signal addition circuit 23. Further, in this embodiment, the transistor 24 whose base is supplied with the once-reflected wave signal e obtained via the delay circuit 21
However, since the amplitude is limited by the base-emitter voltage V _BE2 , the equivalent block diagram of the embodiment is shown in FIG. 12. That is, in this embodiment, equivalently, the once reflected wave signal
Amplitude limiting circuit 33A for e〓 and composite signal e ₀
An amplitude limiting circuit 33B is provided.

As is clear from the description of each of the embodiments described above, according to the present invention, the third harmonic component that causes the inversion phenomenon is suppressed while the high frequency component is enhanced, so that the S/N deterioration is suppressed. It is possible to obtain a high-quality angle-modulated reproduction signal by reliably preventing the inversion phenomenon without any interference, and the intended purpose can be fully achieved.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing the reversal phenomenon. FIG. 2 is a waveform diagram for electrically explaining the cause of the above reversal phenomenon. FIG. 3 is a block diagram showing the structure of a magnetic recording/reproducing system designed to prevent the above reversal phenomenon. FIG. 4 is a frequency distribution diagram for explaining the operation of the magnetic recording/reproducing system. FIG. 5 is a waveform diagram for electrically explaining the cause of occurrence of the reversal phenomenon that cannot be prevented in the magnetic recording/reproducing system. FIG. 6 is a block diagram showing the configuration of a high frequency enhancement circuit in the magnetic recording/reproducing system. FIG. 7 is a waveform diagram for explaining the operation of the high frequency enhancement circuit. FIG. 8 is a circuit diagram showing a specific configuration of the high frequency enhancement circuit. 9th
This figure is a circuit diagram showing an embodiment of the present invention constructed by modifying the high frequency enhancement circuit shown in FIG. 8 above. FIG. 10 is an equivalent block diagram of the above embodiment. FIGS. 11 and 12 are a circuit diagram and an equivalent block diagram showing another embodiment of the present invention. 21...Delay circuit, 22...Matching resistor,
23, 23'...Signal addition circuit, 24, 24', 2
5, 25'...transistor, 31, 32...diode, 33, 33a...amplitude limiting circuit.

Claims (1)

[Claims] 1 Amplitude limitation is applied to the signal with enhanced third harmonic component obtained by adding the reproduced angle modulated original signal and the signal delayed by 2τ time, which is twice the unit delay time τ, and the above amplitude is An angle modulation signal reproducing circuit configured to add a limited signal and a signal delayed by a unit delay time from the original signal in opposite phases to obtain a reproduced angle modulation signal with enhanced high frequency components.